Elusive black holes finally revealed at the centre of our galaxy

Israel Montgomery |Friday, April 06, 2018

This is an artist's impression showing the peanut shaped structure in the central bulge. Credit ESO NASA JPL-Caltech M. Kornmesser R. Hurt Close

The researchers found that star formation evolves by fits and starts. That's because, at visual wavelengths, huge clouds of dust and gas obstruct our vision - we can only see about 10% of the distance to the galactic centre in this way (Sagittarius A* is approximately 26,000 light years away). This makes the system emit X-rays, which we can detect. But they hadn't seen evidence of them in the Milky Way core until now. Using that ratio, Hailey figures that even though they only spotted a dozen there must be 300 to 500 binary black hole systems. These types of black holes are referred to as black hole binaries.

This week, a team of scientists led by Charles Hailey, from the University of Columbia (USA), announced in Nature magazine the discovery of several binary systems in which a part of the couple is a black hole.

A group of astrophysicists analysed a set of X-rays blasted from the centre of the galaxy in a hunt for black holes. "The white dwarf binaries look bluer and the black hole binaries redder".

In the new study, Charles Hailey, an astrophysicist at Columbia University, and his colleagues scrutinized the past dozen years of data gathered by the Chandra X-ray Observatory, an orbiting craft whose instruments are created to detect high-energy radiation emitted by the immensely hot material surrounding exploded stars and near black holes.

At the hearts of most, if not all, galaxies are supermassive black holes with masses that are millions to billions of times that of the sun.

Chuck Hailey from Columbia University and colleagues teased out the faint signal of a dozen "black hole binaries" - where a black hole and another object, such as a star, orbit each other - close to their supermassive counterpart in the centre of the Milky Way.

Despite telescopes being trained on the galactic centre for more than a decade, astrophysicists have had no joy.

Combing through archival data from NASA's Chandra X-ray Observatory, Charles Hailey at Columbia University in NY and his colleagues were able to finally tease out a signal that appears to be coming from 12 stellar-mass black holes that have sun-sized stars orbiting them.

This research would not just shed light on the nature of black holes in the universe, but also on gravitational waves. They form after the spectacular death of a massive star - about 10 times that of our own sun - a supernova explosion that can outshine the star's host galaxy.

GREENFIELDBOYCE: He says calculations based on their new data show that our galaxy's center must be home to around 10,000 black holes.

"If we could find black holes that are coupled with low-mass stars and we know what fraction of black holes will mate with low-mass stars, we could scientifically infer the population of isolated black holes out there".

Black holes tend to come in two sizes.

Stars form as gas and dust succumb to gravity and fusion is ignited. But mostly the center of the galaxy is the ideal "hot house" for black hole formation, with lots of dust and gas. These black holes eventually are believed to congregate around the centre.

"It is going to significantly advance gravitational wave research because knowing the number of black holes in the centre of a typical galaxy can help in better predicting how many gravitational wave events may be associated with them".

But binary black hole systems are likely only 5 percent of all black holes, so that means there are really thousands of them, Hailey said.

HAILEY: Because they're so heavy, they naturally sink or gravitate towards the supermassive black hole in the center.

Either way, our galaxy's supermassive black hole should be surrounded by hundreds of stellar-mass black holes - at least.